Isotope analysis for understanding the hydrogen transfer mechanism in direct liquefaction of Bulianta coal

Direct coal liquefaction (DCL) was investigated using a deuterium tracer method which makes it possible to determine which structural positions in the coal react with H-2 or hydrogen-donor solvents, i.e. hydrogen-transfer mechanism. D-2 and/or 1,2,3,4-d(4)-tetralin (tetralin-d(4)) were used in DCL of Bulianta coal at 7.0 MPa (initial charge pressure) and 445 degrees C for 0 or 60 min in a tubing-bomb microreactor. H-2 NMR and isotope ratio mass spectrometry (IRMS) were used to determine the distribution of deuterium in liquefaction solvent and products qualitatively and quantitatively. The H-2 NMR of solvents after DCL under a variety of conditions showed that the alpha-radical of tetralin is an important intermediate in hydrogen transfer to and exchange with coal, and the de/re-hydrogenation cycle of tetralin is not a dominating approach of hydrogen transfer in DCL. The distribution of deuterium in each structural position and the D/H-1 in preasphaltenes and asphaltenes (PAA) under a variety of conditions showed that both hydrogen atoms in tetralin and H-2 could enter alkyl groups remote from aromatic nucleus of PAA in preference to aromatic nucleus and a to aromatic groups of PAA. The hydrogen atoms in tetralin are more easily to stabilize radicals from coal cracking to form PAA than those in H-2 during the heating-up stage of DCL. Both the kinetic isotope effect and electrophilic aromatic substitution reaction in DCL, indicating the existence of ionic reaction process besides a radical reaction process in DCL. (C) 2017 Elsevier Ltd. All rights reserved.